There are no free/unused bytes in the data format 5. 
Instead of amending the format 5 with a single byte like that, I would suggest an actual “delta dataformat” that would, for example, be identical to the current data format 5, with the difference that the “measurement sequence number” is replaced by a “measurement age indicator”, perhaps at 1 unit = 1 minute (or something).
That way a value of 60 would indicate that “this measurement was taken 1 hour ago”. A tag could then, for example, alternate between format 5 and this delta format on every 2nd transmission (or any ratio) and the client device could then draw its own conclusions regarding the change after having received one of each format. This would also keep backwards compatibility with existing client applications that support only format 5 (or ones that only care about the latest measurement).
With a format like this a tag could even be, for example, alternating between current (format 5), 1 minute ago, 1 hour ago and 1 day ago measurements, and a client application could then display this information, too (after receiving one of each). This would actually be quite similar to what I have done by using Automate on my phone to pull delta data from my RuuviCollector setup and display it in a minimum-priority notification (similar to how Nearby notifications used to work):
