I still don't understand how it supposed to work for old devices. I have a Test VM with an old Windows 2008. It does not have the new ISRG Root X1. Is the only way to get it a Software Update / Windows Update or manual install? Browsing to https://valid-isrgrootx1.letsencrypt.org/ does not work (with Chrome or IE). How does such a client get the new Root-Cert?
Any client will only obtain and trust a new root certificate if it is manually added or included in a software update.
The page you link to is just for testing purposes, it won't install a certificate simply by visiting it.
If a device is no longer receiving software updates, the only option would be to manually install ISRG Root X1.
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