I do not actually use any of the others you listed (well, occasionally bitmap index scans, but it's not a killer). The main difference between 8.x and 15.x for me is the small stream of optimizer features and tweaks (e.g. at some point, it started being able to reorder outer joins).
Extensions in Postgres 8 (and earlier) were also half-baked compared to 9.1 (2011), which added CREATE EXTENSION and other quality-of-life improvements including standardization.
The latest complete implementations are what most people have in mind when talking about them.
Would I pick Postgres 15 over Postgres 8? Yes, any day of the week. But I would also readily pick Postgres 8 over most of today's databases (including e.g. the latest MySQL and MongoDB versions), for most projects, if that was the choice I had.
MySQL foot guns were mostly under control by 2010, and it was operationally efficient to manage a cluster of MySQL servers in diverse configurations with mostly-known boundaries to what could go wrong.
The Postgres foundations are the same today, but what you can build on top is very different today than it was pre-9.x.
This assumes you need replication, though. You can get pretty far without it, especially given how wide hardware you can get now.
> MySQL foot guns were mostly under control by 2010
Eh. :-) I think you would have to specify which footguns in particular.
Also replication can be good for other operational reasons, such as zero downtime major version upgrades. Again depends on the business need/expectations.
Risks to the business are more important to me than my enthusiasm for a technology or product.