Did you know YouTube has RSS feeds? No? Probably because YouTube makes them nearly impossible to find. But here's an easy way to get the RSS feed for any YouTube user. Inspired by <https://eggfreckles.net/notes/youtube-rss/>

youtube-rss.sh

#!/usr/bin/env zsh -f
# Purpose: 	get the RSS feed for a YouTube page
# Inspired By: 	https://eggfreckles.net/notes/youtube-rss/
# Gist: 	https://gist.github.com/tjluoma/fdbc63ceb78a2aecd3d638fd18b6ec6e
#
# From:	Timothy J. Luoma
# Mail:	luomat at gmail dot com
# Date:	2020-01-17; updated 2021-01-10

# 2021-01-10 YouTube currently has both 'rssUrl' and
# 	link rel="alternate" type="application/rss+xml" title="RSS" href="…"
# 	which I do not believe where there a year ago when I first wrote this
# 	I am looking for the latter, and if I don't find it, I fallback to the
# 	former. If neither are found, an error is reported to the user.
#
# 2021-01-10 (Update 2): The previous comment only applies if you are looking
#	at a channel page, i.e. "https://www.youtube.com/512pixels" but NOT if you
#	are looking an an individual video such as 'https://www.youtube.com/watch?v=gQgSdwkvaDg'
#
# Version 2021-01-10.2 -- This script _should_ now work on either of those kinds of pages

NAME="$0:t:r"

if [[ -e "$HOME/.path" ]]
then
	source "$HOME/.path"
else
	PATH="/usr/local/scripts:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin"
fi

COUNT='0'

for URL in "$@"
do

	LINK_REL=$(curl -sfLS "${URL}" \
				| tr '<|>|,|\r' '\n' \
				| awk -F'"' '/application\/rss\+xml/{print $8}')

	if [[ "$LINK_REL" == "" ]]
	then

## Do Not Indent
CHANNEL_ID=$(curl -sfLS "$URL" \
| sed -e 's#<#\
<#g' \
-e 's#>#>\
#g' \
| awk -F'"' '/meta itemprop="channelId"/{print $4}')
## END - Do Not Indent

		if [[ "$CHANNEL_ID" == "" ]]
		then

			echo "$NAME: '\$CHANNEL_ID' and '\$LINK_REL' are both empty for '${URL}'." >>/dev/stderr

			((COUNT++))

			continue

		else

			FEED="https://www.youtube.com/feeds/videos.xml?channel_id=$CHANNEL_ID"

		fi

	else
		FEED="$LINK_REL"
	fi

	echo "$FEED"

			# copy URL to clipboard
	echo -n "$FEED" | pbcopy

done

if [[ "$COUNT" == "0" ]]
then
	# echo "$NAME: No errors"
	exit 0
elif [[ "$COUNT" == "1" ]]
then
	echo "$NAME: One error"
else
	echo "$NAME: $COUNT errors"
fi

exit $COUNT

#EOF

If you just want to extract the URL with an Javascript Bookmarklet, just copy the following snippet in a bookmark link:

javascript:(()=>{alert(ytInitialData.metadata.channelMetadataRenderer.rssUrl);})();

Thanks @tjluoma for the hint in your comments