My own Smalltalk library (and yes, I've tried to put it on github, I
don't know what I'm doing wrong)

If you want we can get with it. Just ask and we can do a videos sessions with zoom,m webex, team or whatever software.

includes Deque and BoundedDeque, both descendants of Collection.
Using addLast/removeLast gives you a stack (use #last for peeking)
Using addLast/removeFirst gives you a queue. (use #first for peeking)

I have found on my disc astc-1711 but I could not find them.

Can you send us the files?

(1) I am puzzled why there are separate FIFO and LIFO classes rather
than a single BoundedDeque.
-- This has implications for performance.
(2) I am puzzled why #withCapacity: is used rather than #new:,
familiar from OrderedCollection.

Good point

-- These two points together make it hard to just swap
OrderedCollection and ?IFOBuffer.

(3) I am puzzled why #clear is used instead of the familiar #removeAll.
-- See note on question 2.

Good point this is nice for API with Collection.

(4) I am extremely puzzled why ALL, like ALL, of the internals of the
data structure are exposed.
Did encapsulation fall out of favour and I didn't get the memo?
(5) It looks as though calling #capacity: at the wrong time can
destroy the integrity of one of
these containers, but there is nothing sayiing "don't do that".
(6) I am puzzled that they are not Collections.

What do you mean.

If this is that they do not inherit from Collection I can understand because the API of collection is large.

Now Alok it would be good to look at ANSI and see a bit the protocol

I attach ANSI