Hi, that's what I would have guessed, but the URL doesn't actually change it at all when I visit filters, it just stays clean with no `#` thingies.

Hii,
You’re right — Odoo’s UI doesn’t always update the URL with the filter state, so just copying the URL with filter_id might not work as expected.
How to reliably open a saved filter programmatically:
Find the filter ID:
Go to Settings > Technical > User Interface > Filters (enable Developer Mode first)

Search your filter by name (e.g., "Shared")

Note its numeric ID (say 42)

Use a URL with filter_id parameter explicitly:

The URL format is:
/web#action=<action_id>&model=<model_name>&view_type=list&filter_id=42
where <action_id> is the ID of the action attached to your menu (can be found in Settings > Technical > Actions > Window Actions).

This URL forces Odoo to load the view with that filter applied.

Oh thanks, that sounds promising! I'll try it as soon as I remember what I actually wanted to link back then :D