@leocg Chrome doesn't have sidebar configuration so these are where Arc store it... same for Opera that has files to store sidebar configs.
That has nothing to do with the profile for each Space.
When you create a new profile to use in Space it create a full new Chrome profile... so inside Arc userdata you have that:
Default (the default profile it comes)
Profile 1 (the first profile you create)
Profile 2 (the second one)
...
Profile N
This is a default Chromium profile folder... that is what is used to different profiles in each Space.
The files there are 99% similar in name to what you found in Opera profile folders... there is some files different but Opera has some of them too.
Arc is not doing something magical... it is just linking a Chromium profile to one Space... just imagine Spaces as being like another window in Opera (with it own profile) but displayed as a workspace instead window.
When you shift Space everything shift with the data of the other profile... folders, pinned tabs, cookies, bookmarks, etc... the StorableSidebar.json probably holds the configuration of the Spaces and the profiles linked... not the info of what is inside the Space / profile (that data is probably inside each profile folder like Opera has a folder for each profile and display the windows with the data of that profile folder).
See here the Arc data folder: https://i.ibb.co/sj7h9Z6/image-2024-08-26-154538664.png
Arc Default profile: https://i.ibb.co/2dXGZB8/image-2024-08-26-154708011.png
Arc Profile 1: https://i.ibb.co/19b3dyh/image-2024-08-26-154807274.png
Opera Default profile: https://i.ibb.co/Tv3nn33/image-2024-08-26-154909746.png
They probably could even link a profile to a Tab if they wished but I think that should create a mess in terms of organization... linking profiles to Workspace is how Workspace should always work since start.
About demand... I guess there is market for that... at least a bigger market than Opera markeshare because Workspace doesn't make sense at all without different session / profiles... the market share of Opera today is very small... any gain is good.