It does seem unlikely Plex would include this ever. Hate their generic pivot to ad supported bs. But as a lifetime sub to Plex I’m riding it down.
I hear jellyfin supports plugins which is one step easier.
At home I watch on an Apple TV with a nas running isponsorblock. Works well but is a single device/location solution. And I am still paying for YouTube premium.
This will be an unpopular answer but I use Nordvpn mainly because during Black Friday sales you can get it essentially for free using a cash back portal. The start of the sale last year, which I missed, was offering over 100% cash back. They’d pay you to use it!
Others are better but if the level of protection you’re looking for is “Comcast stop sending me piracy warnings” this’ll work.