If not, we should set something now and make sure it doesn't break any build scripts, eg 5.10 will be mistaken for 5.1 by some software. For example, it is said that MS skipped Windows 9 because some software confused it for Windows 95.
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Swift is following Semantic Versioning (https://semver.org/), which allows for version specifiers like 5.10 without any problems. It should not lead to any issues at all. If anyone confuses it for 5.1 this is their problem, SemVer is a known best practice. Software that relies on the version for whatever reason should be implemented to work with SemVer properly.
Swift has clear goals for Swift 6, so version 6.0 will be released when these are (roughly) met, I think.
But thank you for sharing your concerns with the community, maybe somebody wasn’t aware of the issue and might need to adjust their code. 
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Unfortunately, the painful lesson of compatibility testing is that even when something is someone else’s problem, it’s still your problem.
Apple folks like me can’t comment on future release version numbers, but this will be good to consider if and when it comes up.
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