Freezing updates is a cool idea. It probably makes sense that any WordPress installation will require some maintenance, which someone needs to be accountable for, or security is seriously at risk.
Possible options are including it in developers maintenance or hosting fee, letting the client manage it.
If a client doesn’t want to maintain the site, maybe a static html site would make more sense, in which case, “
You should be able to use any web site mirroring tool to do it, even from using ‘wget’ from a UNIX box with..
wget -nc -r https://yoursite.com” (https://www.ads-software.com/support/topic/export-to-static-html)