It just dawned on me that the way Diane was written out at the end of season 5 of Cheers may have been a backdoor for Shelley Long to return.

This is just speculation on my part, but when Diane departs, leaving Sam at the altar, she tells him that she'll be back in six months.

Was it written that way so that if Long changed her mind, she could return or maybe they were negotiating with her at the time and wanted to leave the door open for her return?

Shelley Long made it clear she was not renewing her contract and not returning to the show as a regular.

Diane was not killed off so she could have returned at any time if there was a common wish to do so. Shelley participated in the 200th episode celebration special during the 9th season so there could not have been residual animosity and antipathy among the producers to having her back.

The producers must at least have had an inkling that they could bring back Shelley/Diane at some future point if it made sense in a specific circumstance, so yeah they left the door open, but probable nothing more concrete. After all, they could hardly predict how the series would continue to develop, how the stories would arc and even if Shelley would be willing to return whenever they might want her back.
And don't even talk to me about recasting Diane. Ted and Shelley had once-in-a-generation comedic-romantic chemistry that could not have been duplicated,

As Chips says, Diane's exit was left open-ended, even though they knew Shelley Long would never return full-time. The producers talk about this in some of the Archive of American Television interviews on YouTube. (BTW, those interviews are incredible).

Shelley Long has a very limited range, as shown post-Cheers.

I wonder: If Kirstie Alley hadn't caught on with the public and the show's ratings dropped, would the producers, in desperation, have asked Shelley to come back?

Maybe offered her a big buck contract with allowances to be absent from some episodes to make movies?

Keep in mind that in 1987, they were worried behind the scenes that the character of Rebecca might not take with the public and the show would tank.

We now know that didn't happen and the show went on to be an even bigger hit, but I have to wonder what if.

I saw an old interview with Shelley recently that said she was surprised she wasn't brought back for any guest appearances before the final episode. It sounded like she left the show with the expectation that she would be invited back to guest star occasionally. I'd guess that was probably a reasonable expectation. I think when stars left other shows before or during that era, they often did reappear as guest stars for special episodes later.

I can see why the producers decided not to do it in this case though. The show was successful after she left, so they didn't need to look for a ratings boost. And the writing was so good in season 5 and Sam and Diane's relationship so well-developed and explored, how much could they add to it in another couple of episodes?