At this point, the mysql extension has been a "don't use this, you idiot" for nearly a decade. I have no sympathy. None. Choosing to write bad code when examples to the contrary are all over the place means that when somebody finally Does The Right Thing, you will be in for a world of hurt.
Besides, I'm sure some well-meaning soul will come up with some API-identical library or another to drop in. Yes, it'll still take time; consider it penance for screwing up.
But as noted by a sibling comment, it's deprecated instead of being removed. You have plenty of time to not fix your sites, I guess.
A simple shim wouldn't be too difficult as the mysqli extension is very similar to the mysql one. Off the top of my head, you could try something like this for a quick & dirty way:
Besides, I'm sure some well-meaning soul will come up with some API-identical library or another to drop in. Yes, it'll still take time; consider it penance for screwing up.
But as noted by a sibling comment, it's deprecated instead of being removed. You have plenty of time to not fix your sites, I guess.