SIP defines URI decoration in its own way.
After a semicolon, it is called uri parameters.
After the question mark, it is called header fields.
in your case, the first one is correct because Reason is a standardized header in SIP https://datatracker.ietf.org/doc/html/rfc3326 - The Reason Header Field for the Session Initiation Protocol (SIP)
History-Info:<sip:xxxx@xx.com;user=phone?Reason=SIP%3Bcause%3D486>;index=1
The received reason header is copied into the header fields of the history-info URI.
The same apply to other well-known header fields like privacy.
so far, so good but what about the second one ?
History-Info:<sip:xxxx@xx.com;user=phone;Reason=SIP%3Bcause%3D486>;index=1
Well, it is valid too in the context of history-info because it is easier to accept both for compatibility purpose i guess, please see Mapping and Interworking of Diversion Information between Diversion and History-Info Header Fields in the Session Initiation Protocol (SIP)https://datatracker.ietf.org/doc/html/rfc7544
In summary, if the piece of information is coming from a SIP header, it is maybe better to put it in the header fields part of the URI.