I tried my very best to research this question, and I sent the following email to two major Rabbinic bodies in England:
Firstly, I apologize if this is the "wrong address" for these questions.
I was wondering if there was (historically or currently) a standard form for Rabbinic Contracts in Great Britain. I was specifically wondering if there was ever a clause stating that the Rabbi must arrive 15 minutes early to services, and if so, when was it removed, and why. Would anyone at the office of the Chief Rabbi have information about this, or would you be able to provide me with another resource that may be able to help me?
Thank you so much!
The Office of the Chief Rabbinate of England responded with the following:
Thank you for your further email.
Each community or synagogue body has its own arrangements and there is no standard form that applies across the UK.
Thank you for getting in touch with our office.
Therefore, if we assume that Rabbi Rubin meant that there was a standard contract for Rabbis in England, this has changed, and is no longer the case. They did not provide a time that this change happened, nor did they comment on the clause itself.
With this in mind, we can only speculate as to why such a contract was scrapped. Possible reasons may be that more smaller Shuls/Shteibels were opening that were not using the standard contract, or that Shuls themselves wanted more control over specifics or any number of possibilities.
However, it is also possible that Rabbi Rubin was simply saying that in "every" contract, this was a standard request, in which case this does not add anything to the conversation. There could be a number of possible reasons why such a clause may have been removed, possibly due to heshy's reason in his answer, possibly due to less people caring about that, but again, this is all speculation.
(Note that I also reached out to the author on this internet contact form, but received no response. I would encourage others to do the same, in hopes of getting a response.)