This absolutely was the case for a long time. It was the cultural center of the internet where nearly all memes sprang from or gained traction and context before leaving orbit for the greater internet.
That has not been the case for years though. I'd say it shifted to twitter as things shifted to inseparably political on almost all of 4chan maybe 6-8 years back and then shifted away from twitter a while after elon bought it and a lot of people started to bail. and I honestly don't know where exactly it's shifted to now, but I'd have to guess tiktok and similar new platforms.
But regardless I do think 4chan has lost nearly all of it's cultural influence, but still maintains it's notoriety.
I think it's less the case now, but 4chan is absolutely still the source of new slang. It's just less concentrated on that one platform these days.
People were using "Based" and Gigachad in the late 2010s for years before the mainstream picked it up in the 20s.
Also for indie video games, many do find their attention and early fanbases on /v/ before they spread out to twitter. Largely because /v/ is very information sensitive and will pick up primary news usually minutes after they arrive.
‘Slop’ was a 2024 Oxford Dictionary word-of-the-year candidate, and what most of the people using it probably don’t realise is that it originated on 4chan as an abbreviation of ‘goyslop’.