I've never worked on a system where we didn't make an effort to make sure we were NOT dependent on a specific DB vendor/implementation and avoiding SP's has always been a part of that.
We went through and got Oracle out of our enterprise after a particularly nasty licensing negotiation and audit with them some years ago. So it does happen.
I expect at some point we'll have a similar initiative around cloud providers.
Depends on the product in question. For integration products, its very common to support multiple database vendors with the same core engine. So any SQL written and db data-types is generally SQL-92/99.
This is pretty good since one can generally fast unit test DB code with an embedded database.
I’ve done it twice. Mass migration off Oracle and before that DB2 and Sybase. Current system is also occasionally sold on premise to customers who have their own db preference and we can support them all.