In the CoAP specification, it is implied that IEEE 802.15.4 can be used in conjunction with CoAP. Is this a requirement or can CoAP also be used with other OSI layer 1, 2 protocols such as IEEE 802.11, BLE or LTE/5G/etc?
No, CoAP is an application layer protocol it's not dependent
Basically that's the beauty behind the OSI layers. If correctly implemented you can mostly stack them however you want. As with every thing that starts with if correctly implemented that's mostly academic and some protocols fit better together with others than others do. More or less the only restriction is to be able to transfer the data of an upper layer with the lower level protocol.
In the case of CoAP it runs great on UDP which is kind of the intended protocol on the next lower OSI level, the transport level.
Instead of a complex transport stack, it gets by with UDP on IP. — CoAP Website
From our daily Wi-Fi / smart phone experience we all know that IP runs great on 802.11 & LTE/5G.
Bluetooth and it's low energy variant however are actually protocol stacks that go up to the presentation layer. I'm not sure how good the match of CoAP is there directly. It may be easy, but I just don't now.
However with Bluetooth 4.2 they included the IPSP. Basically allowing you to tunnel IPv6 over Bluetooth enabling you to use the standard internet protocol suite from thereon upwards.
The Internet Protocol Support Profile (IPSP) allows devices to discover and communicate to other devices that support IPSP. The communication between the devices that support IPSP is done using IPv6 packets over the Bluetooth Low Energy transport. — Bluetooth 4.2 Specification