More online reservations are possible.

For Spain, some reservations can now be booked via happyrail.com or eurail.com.

In general, check this page for where to book reservations:

https://www.seat61.com/interrail-and-eurail-reservations.htm

Are there any changes to what routes/lines the Eurail Global pass covers around cities like London, Paris etc?

Last time I had a hard time figuring out how to add journeys on the ThamesLink... was trying to get from St Pancras to High Brooms via London Bridge... was able to find a way only through input from this community! 

I still cannot see the journey in the app so was wondering if I can travel on those lines across London like last time?

In London, you can use Thameslink, London Overground (now known separately as: Mildmay, Weaver, Suffragette, Liberty, Lioness, Windrush lines), Elizabeth Line and the commuter rail lines mostly serving South London. 

I think these all show in the Eurail timetable tedit: the Thameslink trains don’t seem to be showing right now, so you may have to add the journey manually] so that’s one way to double check your route is valid.

If you’re using Google Maps or something to plan your travel around London, you’ll need to independently check, because Google doesn’t care whether it routes you via train or tube.

For St Pancras to High Brooms, you would take this route. Both included. (Don’t take the tube to London Bridge)

Thameslink  St Pancras International - London Bridge 
Southeastern  London Bridge - High Brooms

In other words, all these lines (ThamesLink, Elizabeth & Overground splits) are covered by the Eurail Global pass? 

Any Paris City lines (Metro, RER or Transilien etc) included to the Global Eurail pass? Just wondering...

Correct.

None of them. Only the TERs to/from other regions.