Google Ads has completed testing monthly campaign budgets. It is reported by Search Engine Land. Those advertisers who had access to this feature no longer have it. The following notification appeared in the Google Ads interface:

“Monthly budgeting will no longer be available as of April 2020. Your campaign budgets will automatically switch to daily."

Google Ads started testing monthly budgets in September 2019. It seems that this feature did not work out in practice, so the service decided not to launch it for all advertisers.