That's been the limit since sometime between November 29 [1] and December 13 [2]. The daily tweet limit has been 2400 since at least 2018-01-03 [3]. I'm not sure why the DM limit was reduced.

1. https://web.archive.org/web/20221129122631/https://help.twit... 2. https://web.archive.org/web/20221213164704/https://help.twit... 3. https://web.archive.org/web/20180103202353/https://help.twit...