I thought you could only abort if you haven't moved yet or your opponent hasn't when you play white against them.
Why is my account restricted?

I agree that an alert feature could help with this issue.
One thing that you might try is to shift from 5m to 3m and see if that reduces the time that it takes to find a match. My understanding is that there tend to be more players of your strength in that pool.
Another option might be to find a club or external community of strong players and arrange games/matches with them rather than relying on the random pool.
(Being too strong to find quick matches is one of those first world problems that I hope to have someday, but I can understand how it would be frustrating.)

First of all, when I search for a game, it usually takes very long to find a match, so during that period I have to do something else on the phone while chess.com app running in the background. However, when the game starts, chess.com app is unable to send an alert, so I don’t even notice that and thus the game gets aborted after 20 seconds. This is definitely one important thing that the app needs to improve.
Secondly, sometimes I have to abort because I feel the opponent is likely cheating. I have received many messages from chess.com that my rating was restored because someone cheated. It happens very often.
Based on the above, I believe it is unfair that my account is restricted.
Yes, if you abort too often as a percentage of completed games, you will get restricted.
https://support.chess.com/article/627-sportsmanship-policy
First of all, when I search for a game, it usually takes very long to find a match, so during that period I have to do something else on the phone while chess.com app running in the background. However, when the game starts, chess.com app is unable to send an alert, so I don’t even notice that and thus the game gets aborted after 20 seconds. This is definitely one important thing that the app needs to improve.
Secondly, sometimes I have to abort because I feel the opponent is likely cheating. I have received many messages from chess.com that my rating was restored because someone cheated. It happens very often.
Based on the above, I believe it is unfair that my account is restricted.