Am I wrong to think my senior using an expiry date value null as forever weird or is this common practice?

I'd say it's common practice for when you need expiry

Why do you think it's incorrect?

If it’s not going to expiry, why force someone to update it in 5+ years, whom may or may not be yourself.

Otherwise if it’s not a null date, I’ve seen booleans used instead to be a non-expiry item with a date of 0000-00-00...

null, as in "doesn't expire" is pretty common, yes.

It's common. You could however argue to instead use "9999-12-31" as never expire, as db queries will just have to check if "currentDate < expireDate" instead of "(currentDate < expireDate OR expireDate IS NULL)" however querryoptinizers should take care of that. Probably.