Can you buy anything at all on the internet?
If attackers who cannot add a comment to their exploit are in your threat model.
Personally, I've been using a browser extension that blocks JS unless it has a comment reading
> This code is NOT evil or malicious!
at the top. Haven't been hacked yet!
https://9to5mac.com/2021/03/11/browser-based-attack-affects-...
Turns out you don't need Turing completeness to perform microarchitectural side channel attacks. This is yet another way in which the "all my software is free, therefore I am safe from attacks" fallacy breaks down.
Nevermind that, as pointed out by other replies, LibreJS provides zero security. It relies on scripts voluntarily declaring that they're freely licensed, and if they do, they're allowed to run. The extension doesn't care whether the script is malicious or not.
LibreJS is a good example in order to kill any potential Spectre/Meltdown attack. There is no attack when no code is being run.