These aren't errors, they're just warnings. Then again if J3 is supported for a couple more years after J4 is "released", browsers may have already started removing support for Array.forEach and Array.slice, so a core hack may be required or a polyfill

Wouldn't the easier/better fix be to just update the package Mootools to 1.6.0? The base include version of Mootools is 1.4.5 (at least in Joomla 3.9.13). I don't think current versions of Mootools proc any deprecated warnings.

MooTools 1.5 broke compatibility with 1.4 and earlier. At this point it's better to just leave well enough alone than try to do a major JavaScript library/framework update, especially considering the framework's use should be disappearing quickly as it goes away in 4.0.

As stated by michael a b/c break would be worse, if the deprecation get removed finally before J3 is EOL we have to patch mootools.

I'm closing this for now. Thanks