Do rewrites added with add_rewrite_rule() persist after plugin deletion?

Let me say beforehand that when using add_rewrite_rule() nothing is written to the htaccess file. Rewrite rules are stored in the database and handled by WordPress internally on PHP level.

TL;DR: Rewrite rules do persist after plugin deletion, but probably not the way you’d expect it.

When using add_rewrite_rule these rules are added in the options table in the database. You then need to flush the rewrite rules via flush_rewrite_rules() (Visiting Settings > Permalinks and saving does the same) for them to come in effect.

Similarly when you remove your plugin that added the rewrite rules they stay in effect until the next flushing of the rewrite rules. So yes they persist, but they will disappear at some random point later on when the rewrite rules are flushed.