Question about how a bank knows that a check has cleared - how do they know?

I generally understand the financial system and how ACH works, but I have always wondered how a bank knows that a check has cleared.

Example:
I use Bank A as my normal bank. Someone sends me a check for $10,000, drawn on Bank B. I take it to my bank and deposit the check in full.

My bank, Bank A, places a hold for all but $200, because they don’t know that the check is going to clear. (This has been common practice for decades). The hold last for some number of days, usually 5, I believe, and then I get full access to my money.

As I interpret this, Bank A never gets positive confirmation that the check cleared and was paid by Bank B. Bank A uses negative confirmation: “the check didn’t bounce in 5 days so it must be OK.” (of course, the question here could be: banks reconcile their transactions overnight, so why do they need 5 days?)

So here are my questions:

a) in this day and age of instantaneous communications, why doesn’t the originating bank (Bank B in my example) send back a message saying “paid” or “bounced”, to Bank A? Why does this need to go through the archaic negative confirmation process?

b) What ever happened to that initiative called Check21 that was purported to turn paper checks into electronic checks, thereby speeding up the process?