For details see:
http://lists.squid-cache.org/pipermail/squid-users/2023-July/025929.html
"The Squid HTTP Proxy team is very pleased to announce the availability
of the Squid-6.1 release!
This release is we believe, stable enough for general production use.
Support for Squid-5 bug fixes has now officially ceased. Bugs in 5.x
will continue to be fixed, however the fixes will be added to the 6.x
series. All users of Squid-5.x are encouraged to plan for upgrades."