I have searched the site and found there is already a question about this: Accepting answer without upvoting? but it is already 4 years old.
In my opinion, Auto-upvote should be in place and here is the full solution, does anyone know why this is still not in place? :P
Improve even without Auto-Upvoting:
- For new users (thanks to @psubsee2003) (rep. less than * or never upvoted..) Highlight the upvote function and show tips like "you can upvote the answer if you think it's helpful" upon user accepting an answer.
- Send reminder to user without enough Rep/vote, to upvote an already accepted answer, when they get enough Rep/vote.
Possible problems with Auto-Upvoting:
- User don't have enough Rep to upvote
- User don't have any vote left
- User don't really want to upvote
- Auto upvote the question that is accepted when they do get enough Rep. and send them a notice asking if they want to undo the upvote.
- Auto upvote the question next day, and send them a notice asking if they want to undo.
- Highlight the upvote and user can undo it upon marking as accepted. (this is not a problem compared with how user will never know they can upvote&accepted).