As has been noted previously, postfix has displaced sendmail as the MTA of choice in Slackware, with sendmail relegated to extra/. However, there are evidently a large number of Slackware users who are utilising sendmail with relatively complex configurations involving milters and custom rulesets which have been refined over the years. Porting these to postfix will take considerable time which is hard to justify given that sendmail still has active upstream support.

To ensure that sendmail doesn't quietly disappear, could a move back to the "n" series be considered prior to the release of Slackware 15.0? Postfix could still remain the default option, and there are good technical reasons for this. However, having sendmail in the "n" series alongside postfix provides some degree of certainty for those who rely on sendmail - that it won't be dropped unexpectedly in the future.

There is a precedent for such an approach: Slackware includes both proftpd and vsftpd. I have no problem with this because there are some classes of FTP servers which are easier to configure with proftpd while others are best implemented with vsftpd. Similarly, while postfix is an excellent choice for perhaps most mail servers, there are some situations where sendmail is still the better option.

I suppose sendmail has been moved to extra/ because some of the binaries overlaps with those installed by postfix.
With both packages in n/, each package would overwrite parts of the other at every install/update.

That's fair: the postfix package evidently installs a component of itself as usr/sbin/sendmail (for obvious reasons), and there may be other files affected similarly. It is a touch awkward. Perhaps that is the reason they aren't both present in "n". However, the default install could flag one package for install (postfix) and the other for skipping (I think the shipped tagfiles can do this), which would minimise the chances of someone installing both. The overlap is clearly unfortunate though, and conceptually messy. That said, it would be good if some sort of solution could be found for the reasons stated in the OP, even if this is simply some sort of statement that sendmail in extra/ won't go away any time soon.

It won't be going anywhere as long as upstream continues to maintain it.

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Thanks for the definitive confirmation. This is welcome news for those of us with a lot of time invested in sendmail configurations.

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I don't use sendmail anymore and have moved to postfix but we appreciate the clear message Pat.