The prototype implementation (which was shipped in 2020 and then shape-changed in 2023) contained a method called `getInnerHTML()` that could be used to serialize DOM trees containing shadow roots. That part of the prototype was not standardized with the rest of declarative shadow dom, and only recently has it reached spec consensus (https://github.com/whatwg/html/issues/8867). As part of that consensus, the shape of the getInnerHTML API changed. This feature represents the deprecation of the old, shipped `getInnerHTML()` method. The replacement is called `getHTML()`, and it shipped in M125. See https://chromestatus.com/feature/5102952270528512 for that feature.
The standardized method, getHTML(), will likely be interoperable (once implemented in other browsers), while this version (getInnerHTML) will not.