"Is Ikesoren canon" is a much more complicated question than a lot of people will give it credit for, namely because of all of the different layers of canon.
Layer 1: Obvious Canon. These are the things that anyone playing the game could see from standard cutscenes and base conversations including but not limited to:
Ike prioritizes making Soren happy over doing his job (PoR 13), doing his job over looking at flowers (PoR 17), and looking at flowers over looking at women (PoR 14)
Ike finds Aimee off-putting (PoR 13 and 23, RD 3-2 and 4-1)
Soren and Ike worry about each other constantly and protecting each other, including but not limited to Ike placing himself in front of Soren to protect him from a tiger attack (PoR 8) and Soren lashing out when he can't follow Ike to fight the Black Knight (PoR 27) as well as the bond bonus.
Soren knows how to woo Ike (PoR 23)
Ike, in no uncertain terms, does not want to marry Elincia (PoR epilogue and RD)
Micaiah and Yune both refer to Soren as the boy who is always beside Ike (RD 4E)... Confirmed when fighting Ike as an enemy (RD 3-13).
Ike is not (just) oblivious to love (RD 4-1)
Layer 2: Esoteric Canon. These are things that require extra effort to see but are either implied to always happen or have no long term consequences. This includes the entirety of Ike and Soren's PoR support path as well as Soren's battle quotes with Shinon and Micaiah and escape quotes. Here we also add...
Soren being able to recognize tiredness on Ike's face through a small tic Ike doesn't notice and Ike compliments him afterwards (PoR C)
Soren expressing that Ike is the only person he can rely on and Ike assuring him he will always stand by him (PoR A)
"There's only one place for me to be, Ike, and it's by your side" (PoR epilogue, if A supported)
A warm core is melting Soren's freezing heart according to Micaiah (RD 3-7)
Layer 3: Uncertain Canon. These are things that most likely happened according to game preferences, but can be avoided. This would include Ike and Soren's Radiant Dawn supports and paired ending as well as any new game+ exclusive material. Most of the nature of Ike and Soren's relationship is here, including:
"I vow to remain by your side as long as I draw the breath of life" (RD A)
"My only wish was to see you again" (RD 4-E)
The support ending where Soren sets off with Ike and Ike was never seen again
Layer 4: Implicit Canon: Sometimes things that should be obvious are not spelled out, so a lot of the disagreement happens between these two levels. Here are some good examples:
When Micaiah talks about warm feelings, she means romantic ones because both her and Sothe, and Naesala and Leanne, whom she also uses the terms for, explicitly get married. Soren saying "do not presume to understand me" is a dodge, not a denial.
When Ike says Aimee "has issues with reality", that is in direct reference to her theory that he's (just) not ready to settle down.
All of the Aimee, Elincia, and temple handmaiden evidence suggests that Ike, at the very least, does not prefer women.
Because Ike and Soren are emotionally closed-off, them sharing a tender hug (technically also an inference but not a controversial one) says more than it would for the average characters.
Ike and Soren leaving alone together forever heavily suggests a romantic relationship.
Layer 5: Word of God: At this point, canonicity starts becoming more nebulous. This could include anything from side materials such as the art book, which says that "Ike's sense of how dear Soren is to him is renewed" and that "Ike doesn't care for the affections of women." Both are already present in implicit canon. But this makes them explicit, if only outside of the work.
Layer 6: Improbable Canon: These are things that are present in the game, but require you to make choices counter to the best narrative. This will include any of the death quotes, for which all of Soren's and two of Ike's reinforce how much they care about each other. The one-sided reading could potentially fit here as well, as so much of Ike returning Soren's affections requires them to have a paired ending (and special scene if possible), so the two less-encouraged Ike endings (and by extension, one DIScouraged Soren ending) could come across this way.
Layer 7: Soft Canon: Anything that is supported not by PoR and RD or their teams, but is supported by FE crossovers such as Heroes, Engage, and the Awakening DLC. And yes, this does mean that Priam is more nebulously canon than Ike and Soren's support ending. Sorry not sorry.
Layer 8: Non-Canon: Headcanons that do not contradict any official evidence but aren't supported by it either. While some people will have these to connect Ike/Soren to Priam and/or Soren's relatives, to speculate on their future, or to further characterize either, this is NOT necessary to think Ike and Soren are a couple in general. This would also include more general crossovers such as Smash Bros.
Layer 9: Counter to Canon: finally we have this set. These are definitively untrue based on level 1 evidence and the aggregation of levels 2-6, but people will use them for a variety of reasons from trying to relate better to the characters to write a what-if scenario to... My entire "straight Ike arguments" and "straight Soren nonsense" tags.
Conclusion: Ike and Soren having their primary relationship is layer 3 at worst, it being romantic is layer 5 at worst, and being adamantly opposed to it is level 6 at best and 9 at worst.
