The details were provided in the changeset comment, to which you could have replied.

Sidewalks which do not connect to anything are *not* useful for creating running/walking routes. They're at best decorative and at worst spam.

The sidewalks added by you which connect to other highways are still there, together with sidewalk:$side on the parent highways.

I walk and run a lot in London and have added a lot of crossings and separate sidewalks. These are accurately mapped, actually work for routing and include accessibility features. Even after adding them with the best available aerial and street-side imagery, it's still worth walking the newly-added sidewalks with StreetComplete to add missing details.

Incidentally, in what way is London your local area? It's only just on the same continent.
https://hdyc.neis-one.org/?asturksever

Pond tags restored. As effectively deleting the pond this was not mentioned in your changeset comment, I have assumed this was unintentional (and not the deliberate deletion of an existing feature, which is generally considered to be vandalism).

changeset/149765428

As you have not responded, this has been reverted in changeset/149765275

Fixed in changeset/149765163

Decorative sidewalks deleted in changeset/149764140

There is no point adding a separate sidewalk unless it is correctly connected to other highways, usually via crossings.

The sidewalk tags on the parent highway should also be updated.

Re-tagged as an unmarked crossing in changeset/149584087

Fixed in changeset/149763083

The crossings here are correctly mapped by local mappers who have (a) been there and (b) know what they are doing.

There is no crossing where you added this node. Nonsense reverted in changeset/149762799

It's only worth mapping separate sidewalks if they actually connect for routing purposes, generally at crossings. They're just decorative otherwise.

Footways removed in changeset/149751509

Thanks - I spotted that when I opened it in Vespucci. Should be fixed in #149645891

Fixed in changeset/149601330

Please could you add crossing ways as footway=crossing, not footway=sidewalk. Thanks.
footway=crossing

It also helps if the crossing node is actually added where the crossing intersects the road being crossed.

Fixed in changeset/149601014

Please add crossings with signals as crossing=traffic_signals, not crossing=marked. It's also generally considered helpful if you put the nodes the correct side of the highway=traffic_signals node.

Cleaned up in changeset/149589908

Please could you add crossing ways as footway=crossing, not footway=sidewalk. Thanks.

footway=crossing

Is there any particular reason why you have made Kotree Way unreachable from St James's Road?

The Bing street side imagery (which may be out of date) appears to show a connection to St James's Road at the end of the railings, next to the Esmeralda Road bus stop.

Having the crossing tags on a footway which is not connected to anything else may not be quite as helpful as tagging the crossing node itself.

Tags moved to crossing node in changeset/149584746

Tidied up in changeset/149584481

If you're adding separate sidewalks, please check and update the sidewalk=* tags on the parent streets.

Updated for this in changeset/149583280

I'm pretty sure I mapped this correctly when I surveyed it in real life.

See crossing:markings=*#Examples

Reverted in /changesets/149578731

Please don't change crossing=traffic_signals to crossing=marked