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By doing that you have excluded theme plugins from templates compilation. This is temporary solution until next recompilation.
There is an issue with PHP8 uncatchable error, which was deprecated in PHP7.4. Some theme plugin hooks use trick with parent template function overloading in addition to documented hookData() method:
public function includeCSS() { \IPS\Output::i()->cssFiles = array_merge( \IPS\Output::i()->cssFiles, \IPS\Theme::i()->css( 'somePlugin.css', 'core', 'front' ) ); return parent::includeCSS(); } As a workaround you can fool the interpreter by changing 
return parent::includeCSS(); to
return \call_user_func_array( 'parent::' . __FUNCTION__, func_get_args() ); The difficulty is that these new errors are not catchable by error handler, so you can't know which hook file is causing them. This often happens with this plugin https://invisioncommunity.com/files/file/9966-back-2-top/ and this app https://invisioncommunity.com/files/file/7940-iawards/
 

By doing that you have excluded theme plugins from templates compilation. This is temporary solution until next recompilation.
There is an issue with PHP8 uncatchable error, which was deprecated in PHP7.4. Some theme plugin hooks use trick with parent template function overloading in addition to documented hookData() method:
public function includeCSS() { \IPS\Output::i()->cssFiles = array_merge( \IPS\Output::i()->cssFiles, \IPS\Theme::i()->css( 'somePlugin.css', 'core', 'front' ) ); return parent::includeCSS(); } As a workaround you can fool the interpreter by changing 
return parent::includeCSS(); to
return \call_user_func_array( 'parent::' . __FUNCTION__, func_get_args() ); The difficulty is that these new errors are not catchable by error handler, so you can't know which hook file is causing them. This often happens with this plugin https://invisioncommunity.com/files/file/9966-back-2-top/ and this app https://invisioncommunity.com/files/file/7940-iawards/
 

sorry Daniel, but this is NOT a third party issue. This is an engineering problem inside the framework. This was an issue reported/showed up almost 2 years ago and u guys did nothing to fix it or even comment on it, it was left up to me to debug it and come out with a solution. This is a good example of us using features of php (and of IPS) and since the framework is still mostly a pho 5.3 framework, it not playing nice with the modern versions. This is honestly the kind of nonsense of why I left.
These are not third party developer issues, don’t let IPS TELL U THEY ARE. these are issues caused by an engineering problem with in the framework as demonstrated by the topic Adriano linked too. They are eval’ing a non extended class to check for errors or something like that and that is where it throws the deprecate notice in 7.4 and the fatal in 8.0+. Put IPS’s feet to the fire, demand better from a software company that charges $300 a year for renewals for mediocre improvements and many years of outstanding bugs they ignore or refuse to fix.