My ISP has a 1Gb limit and blocked my site.
The file grew to 16 Gb in less than a week. I emptied the watchdog sql file in the database with phpAdmin. This overcame the block and the site restarted, with an SQL error on the front end and hundreds of SQL errors on the back end, which still, nevertheless, will start.
In less than 10 minutes after emptying the file, the size had returned to 1 Gb. 45 minutes later, there were 250,000 rows occupying 3 Gb.
The site is normally quiet. It has had little in the way of addition in the last 6 months and serves about 200-500 pages a day.
The logged error is the following.
Deprecated: assert(): Calling assert() with a string argument is deprecated in Drupal/Core/Plugin/DefaultPluginManager.php on line 151