@rrrooommmaaa build on macOS failed because of m flag. I removed it and seems everything works well even without it

The patch only matters for Firefox, and Firefox is not tested automatically, so we can't tell whether it works well.
Does this statement actually patched the file -- does the new file differ?
The function isUint8Array should look like this:

function isUint8Array(input) {
  return Uint8Array.prototype.isPrototypeOf(input) || globalThis.Uint8Array.prototype.isPrototypeOf(input);
}

The m flag is supposed to give ^ meaning "beginning of any line" instead of "beginning of file"

I re-checked - files were not patched without m flag.
But I updated ISUINT8ARRAY_REGEX, so it's not using ^, and now files are patched.
What do you think about updated regex, is it ok?

I think it's ok to not use ^,
It'd be nicer to leave "one or more whitespace" before return with (\s+) and \1 in this case.
What is wrong with sed on MacOS? Is it some old version? Can't it be updated?

I updated regex to use (\s*) (as it didn't work after changing to (\s+))

What is wrong with sed on MacOS? Is it some old version? Can't it be updated?

Yes, macOS includes an older version of sed which doesn't support some functionality.
It can be updated by installing another version and then linking it to the default sed command.
I thought about adding this info to our README, but as this ISUINT8ARRAY_REGEX patch is temporary (before openpgpjs/web-stream-tools#23 is fixed) I think updating regex should be ok for now.