browser.sessionhistory.max_entries has no effect
Prior to version 9.* users could change browser.sessionhistory.max_entries to restrict history length for every separate browser tab, by setting a value lower than the default 50. This allowed, on one hand, to prevent additional data being collected about the user - namely, how many pages they visited in this particular tab - and on the other, somewhat cut down on memory consumption.
Tor Browser version 9 and higher ignore all browser.sessionhistory.max_entries values below 50.