"since developed" is correct and usable in written English.
You can use it to describe something that has changed over time, as in "Since developed, the town has experienced a significant increase in population."
An amusing pattern has since developed.
(It has since developed a cult following).
The sessions have since developed into an extensive programme.
It has since developed into an arts camp.
He has since developed a passion for the sport.
That mission has since developed into a much larger contingent of United Nations peacekeepers.
She said Lambert and his son had since developed a close relationship.
McNabb has since developed into one of the most exciting quarterbacks of the last decade.
Last year, Walters' son, Gavin, was diagnosed with lead poisoning and has since developed speech impediments, she said.
It has since developed a number of community programmes, including a local primary school that caters for 600 children.
Szpilka has since developed into a fairly skilled boxer, but he was still an underdog against Wilder.
