You're overthinking things. And GDPR hasn't been an issue for forums.
The sites were abandoned (likely not profitable enough) and the forums weren't worth put. Sure they were worth quite a bit to us, but monetarily they were worth very little. And then add in server costs and they might even have been slight losses.
TechTarget seem to be going as usual, so to echo others here, someone sympathetic to the forums probably left and/or an accounting review found the forums still existing and costing money, so they were axed as part of normal 'fat skimming'.