Yes. You will be notified of new versions (once per new version) before Sigil opens, unless you populate the SKIP_SIGIL_UPDATE_CHECK variable with a non-zero value. There is also a compile time define that can be set to permanently disable the update check. But it's only an update check/notification. There is no built-in auto update feature/procedure.
It's mostly there so distro maintainers can avoid having their packaged versions of Sigil notifying users about new versions of Sigil they may not provide in their repos just yet.
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