Was there a spam in Danbooru during late 2019?

Not sure if coincidence but I notice there is lack of user with ID 600000ish compared to 500k and 700k. I look up in my browser history and yes, the highest I find is https://danbooru.donmai.us/users/62 xxxx which was made on mid June 2019. The only big event in internet I know that time was e-h crisis

Yeah, in 2019-07 we had a pretty major case of spambot attack. Some 70k automated registrations. See this graph of user registrations per month in the last 10 years for reference.
The user names were all using the same random 6-character alphanumeric pattern (like user #671113 for example). IIRC those accounts were all banned server-side, so there's no risk of them ever resurfacing.

Since then, a number of different measures have been implemented to prevent this kind of mass bot registration.