When did Big Finish go off the rails?

I haven't bought anything from Big Finish in years and would like to get back into their better material

I was dismayed to hear hear that they're even shoehorning NuWho crap and SJW propaganda into Classic Who audios these days. I know that 2015 is when they acquired the NuWho license, coinciding with the show's SJW propaganda becoming much more overt in Series 9.

Would I be correct in assuming that anything pre-2015 is safe?

Yes it has gone off the rails. Pre nu who was already in trouble though due to the omnipresence of Nick Briggs and Lisa Bowerman.

They brought back Terrahawks. They're not all bad.

I used to really enjoy Big Finish audios but over the years the tales have grown bland, predictable and lacklustre.  In recent times, I've only enjoyed a couple of Fourth Doctor/Romana stories (including the smashing New Adventures adaptations) and the rest of what I've listened to has been run-of-the-mill stuff and quickly forgotten.

However, though I now find the range tired and pointless (and increasingly desperate), Big Finish is still closer to what I consider to be 'proper Doctor Who' in comparison to the embarrassingly bad modern telly series.

Just confirmed today Alan Barnes will be stepping down as script editor of the main BF range

I'd say it was back in 2010 when they produced The Four Doctors as a subscribers only release, and announced it would never get a general release.

They'd had some crap releases before then under Nick Briggs' new management, like The Dark Husband, and Dalek Empire 4, but that I think was the point where they really showed their contempt for their buyers.

Probably around the period when they released the Sirens of Time.