I haven't spotted any mass deletions of buildings, which hopefully would be flagged by OSMCha. If there are any areas where you're reasonably sure you added buildings which are now missing, it should be possible to find out - see osm.wiki/Overpass_API/Overpass_API_by_Example#OSM_data_at_a_certain_date

Apart from "the editor suggested it and you did it without question", why did you delete crossing=unmarked here? Not only is aerial imagery available showing that there aren't any crossing markings, but unlike you, I've actually been there and surveyed it.

node/9926313735

(Review requested)

Looks fine to me, thanks for updating it.

Are you sure that City of Westminster College is operated by Westminster City Council, because they're under the impression that it's United Colleges Group.

way/288304682

Blindly accepting the suggested "upgrades" suggested by Rapid/iD is not QA and it's not fixing issues. It hides potential issues and created new ones.

When iD suggested a tag "upgrade" adding operator:type=private to an NHS hospital, why did you accept this when it was obviously wrong?

I've raised an issue for iD making the suggestion to "upgrade" Great Ormond Street Hospital by adding operator:type=private. However, a poor suggestion from a QA tool is not an excuse to add information which is obviously wrong.

https://github.com/openstreetmap/id-tagging-schema/issues/1528

I also see that you've added operator:type=private to Great Ormond Street Hospital. Was the error here yours, or a defective suggestion by the iD editor?
way/548533106

What's the point of adding crossing:markings=yes, other than "the iD told you it was a good idea"? If you can see what the markings are, please tag appropriately. Telling data consumers that "this marked crossing is marked" isn't particularly useful.

(Reverted, obviously)

Also, please explain why you believe the source which you failed to understand has a licence compatible with OSM:

"All content on this website ©1996-2016 Nuffield Health or used under licence. This website is protected by copyright. It is published by Nuffield Health and may not be reproduced other than when downloaded and viewed on a single device for private use only. It is not to be otherwise reproduced or transmitted or made available on a network without the prior written consent of Nuffield Health. All other rights reserved. "

https://www.nuffieldhealth.com/terms/nuffield-health-website-terms-and-conditions

Where did you get the idea that the entire Barts Hospital site is operated by Nuffield Health, not the NHS?

I had a quick look and it looks fine.

If you'd like, I can undelete the original track which you added and add those tags.

Deleted again in changeset/165210310

Referred to DWG.

The paths were already tagged correctly with foot=private, so adding access=no was pointless. At least this changeset was mostly harmless, unlike your others.

Reverted in changeset/165221258

(Review requested)

You need to add a tag to tell data consumers what sort of object this is, which in this case is highway=track

You could also add tags describing the width (in metres) and surface type, see:
highway=track
surface=*
tracktype=*
width=*

@BCNorwich see also changeset/165085015

I see that you have chosen to ignore the comment made on your earlier deletion of these paths. I suggest that you read that comment again and also the linked wiki pages.
changeset/165074768
osm.wiki/Why_we_won%27t_delete_roads_on_private_property
osm.wiki/Why_can%27t_I_delete_this_trail%3F

No problem.

There are things like floating gardens in some water bodies which don't render properly now matter how they're tagged. Unfortunately, it's probably a bit too niche to bother with raising an issue.

The problem with adding access=no here is that it doesn't represent the signed restriction and it creates a pedestrian prohibition which doesn't exist.

A no entry sign means "no entry for vehicular traffic", which is vehicle=no (or vehicle=private if you want to include service vehicles at a bus station). The plate with "Except buses" then gives the bus=yes tag overriding vehicle.