It's not that ULA is deprecated, but it discouraged, and this has led to implementation hurdles, like the fact that if you have a device with ULA addresses and

It's not that ULA is deprecated, but it discouraged, and this has led to implementation hurdles, like the fact that if you have a device with ULA addresses and GUA addresses, then your device will originate its requests, even to ULAs, from its GUA. So now your firewall rules need to specify the GUA in the source address field, for example, or you need to reconfigure every client directly. (Also if you're running your services on the GUA, then the destination address also needs to reflect that in the firewall rule).