I don’t believe any long term trends should be read into this by-election, which was contested under a highly unusual set of circumstances.
It looks to me as though the “Get Starmer Out” faction calculated that their best option was to vote for Burnham. That way, Starmer is gone by the end of the summer and possibly earlier. A vote for Reform would have delayed it for three years.
Restore getting 6% of the vote when only a few weeks ago hardly anyone had heard of it is one point of interest. Although I think the party will in reality never be anything much more than a pressure group. Talk of it taking over from Reform and the Conservatives is fanciful in the extreme.