On July 15, 2017, ~36 million posts were made on Tumblr [1]. A year later, July 14, 2018 [4], about ~31 million posts were made [2]. July 13, 2019 saw only ~21 million [3].

The Wayback Machine doesn't have fine-grained enough details to get a good estimate of how much the loss of adult content itself was responsible, but this does reinforce the impression given from the Google Trends link cited several times that there has been a steady decline in Tumblr punctuated with a massive hit on or around the removal of adult content.

[1] https://web.archive.org/web/20170711122839/https://www.tumbl...

[2] https://web.archive.org/web/20180714223645/http://tumblr.com...

[3] https://web.archive.org/web/20190713180936/http://tumblr.com...

[4] Each of these dates is a Saturday, so not quite a year later, but matching time of year and day-of-week is better anyways.