Potentially controversial opinion but the way to reduce the number of fics being deleted and to increase the number of fics that you want to read is to:
Support WIPs if you like the concept, even if it's just with a bookmark (bonus points for reading along and commenting on every chapter, even if it's just with emojis)
Comment and tell the writer you love it and WHY, or just express thanks for them writing it
Share the fic with others and encourage them to comment
Appreciate fic writers instead of taking them for granted
Recognize that fic writing is gift culture and not content creation and that writers get zero compensation for writing other than interaction with readers
Request the kinds of fic/dynamics you'd like to see rather than demanding anything. (IE: You write so and so together so well! Have you ever thought of exploring X? Or: This backstory is so interesting. Would you ever flesh it out more? Or: I'd love to see your take on X.)
NOT
Guilting writers for deleting a fic and saying it's a crime or that you feel "cheated" if someone takes down their work
Making a writer feel guilty because some hypothetical somebody somewhere who never told them so might have liked that work
Criticiizing fic to the author's face or in public (either constructively or not) when feedback wasn't explicitly asked for. (Discussing the merits of a fic in private is fine)
Only shouting about fic in private and never telling the author you love their work
Only reading completed fics and never commenting/otherwise supporting. There are plenty of valid reasons to only read completed fics, but realize that if you don't support fics as they're being written, you will get less of the kinds of fic you like and fewer fics will be completed
Generally acting entitled to fic
Whining that you hate when fic writers do X or that there isn't enough of X dynamic or otherwise trying to dictate what authors should and shouldn't write
Just be positive, folks! Fanfic is a community. I will churn out thousands of words a week if people tell me they're enjoying it. If I get no interaction, that fic is a lot more likely to never be completed or even deleted.
You don't have to unconditionally support all fic writers. It's totally fine to just to kind of like a fic and not comment (though if you read it all the way through, I think at least a kudos is warranted.) But if you DO love it, please please please tell us so, even with just a heart emoji.
The silent enjoyment of fic with no expression of that to the author is what slowly kills fandom - as does positioning us as content creators who get something simply by posting, or by criticizing what and how much we write.
This is especially important if the fic is a rarepair, or a weird concept, or otherwise offbeat. If it's your kind of thing and you don't see very much of it, shout about it! I've definitely deleted or abandoned fics that were a little weird because I felt embarrassed by the fact that no one else was matching my freak.
