Both 6.2.1 and 6.3 Connection Reference online documentation "CHAPTER 5. STOMP PROTOCOL" says:

IMPORTANT
Apache ActiveMQ implements the Stomp v1.1 specification, except for the treatment of spaces that appear at the beginning or end of message header keys. The ActiveMQ implementation of Stomp trims leading and trailing spaces in message header keys (but preserves leading and trailing spaces in the header values). This behaviour is liable to change in a future release.

A-MQ 6.2.1 doc link:
https://access.redhat.com/documentation/en-US/Red_Hat_JBoss_A-MQ/6.2/html/Connection_Reference/AMQConnectRefStomp.html
A-MQ 6.3 doc link:
https://access.redhat.com/documentation/en/red-hat-jboss-a-mq/6.3/paged/connection-reference/chapter-5-stomp-protocol

And we also have a Knowledge base article:
https://access.redhat.com/articles/375743
The "Transport Protocols" section listed supported STOMP version for all of JBoss Fuse versions as 2.0:

In fact, the STOMP version 2.0 is still in draft.

I have confirmed with Gary Tully and Rob Davies that currently ActiveMQ implements STOMP version 1.2. It is true for both JBoss Fuse/A-MQ 6.2.1 and 6.3.

So we need to correct Connection Reference documentation for both 6.2.1 and 6.3, as well as the Knowledge base article to avoid confusion.