PHP8 issues

If you got some snippets in your child-theme functions.php – you had to look for:

$(window).load(function(){
// It is deprecated in php8 ( because jQuery is now 3.5.1 and later)  now and it must be used:
$(window).on('load', function(){
// this here too:
.click(function () {
// had to be replaced with:
.on('click', function () {

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php8 comes along with jQuery 3.5.1 and these are such common stumbling blocks that experienced web designers think about before updating. ;)

Also, a popular mistake is updating the themes but not updating the child-theme substitutes you wrote for the older version. This is what happened with the footer.php. Some new features were added there, which then could not work with an older child theme version.

My advice in this context is to turn off the automatic updates anyway and keep up manually.
I now had the case with a customer that the provider upgraded to Debian Bullseye (09 October 2021) . This would not be a problem except for the fact that Bullseye actually only comes with Apache 2.4 commands. So suddenly modules were missing that some plugin (still) needed. All things you have to read before to update afterwards. Or – you have a test site on which you can fathom such things wonderfully.
Customer sites – especially those with store systems – I always completely secure before with Duplicator Pro – to be able to perform a rollback if necessary.