I've made a simple userscript (Tampermonkey, Violentmonkey, etc.) to add a watchlist/saved search option to e621.

Current Version:
GreasyFork

Features:

• "Saved Searches" button under the search bar. When clicked, will open a dropdown menu with:
• a list of hyperlinked titles (Saved Searches),
• toggleable link edit & delete buttons,
• input field for new additions,
• and Export/Import function for the list

The links can obviously be anything (If you really want to put your grocery list on e621, you do you), but the intended use is for:

• Saved Searches (https://e621.net/posts?tags=happy_sex+pregnant),
• and Pages (https://e621.net/posts/4005902, https://e621.net/wiki_pages/204, etc.)

Planned changes:

• Link Folders (i.e. dropdown lists)
• Currently the entire list is shown, which can be excessive with longer lists

If you'd like to suggest additional features, or contribute code snippets, reply here or at Greasyfork, and I'll see about adding them in (I think the current license accounts for multiple contributors?).

The "Planned changes" section above covers features I plan for the foreseeable future, but I have some other features planned for a future "v2.0", or earlier, release:

• Full e621 site style matching (Buttons, animation, element styling, etc.)
• Ideally dynamic style matching for future proofing
• Link colouring options (author, lore, etc.)
• Sort links by dragging

For now, the Import/Export feature covers sorting and editing (Bit time consuming, but fairly simple).

Changelogs:

v1.4.1 Changelog:

• Fixed Reference Error in Edit Button, should now actually edit Links
• Added comments explaining the code
• Pretty barebones for now, may change comments to be more descriptive in the future

v1.4 Changelog:

• Added button to quickly add current page to list
• Added buttons to edit links in list (Show buttons by pressing "Toggle Link Edit")
• Opens prompt window with current title and link pre-filled
• Simply edit, then select ok to submit changes

v1.3 Changelog:

• Added button to toggle the visibility of the link removal buttons
• No more risk of accidentally deleting links

v1.2 Changelog:

• Added buttons to remove links (x)
• Updated link adding:
• Changed from separate fields for Title and URL to Area Input
• Now uses syntax (Title, URL), separated by lines, for bulk input

v1.1 Changelog:

• Expanded scope to post pages (i.e. https://e621.net/posts/4005902)
• Updated the JSON Export/Import Buttons:
• "Download as JSON" changed to "Export List JSON"
• Default "Choose File" button changed to custom button "Import List JSON" to match "Export List JSON" button style
• As a result, the dropdown list no longer partially covers posts