Track Feature: Cell spanning (aka. Merged cells)
ag-grid: Yes (Free) https://www.ag-grid.com/javascript-data-grid/row-spanning/ https://www.ag-grid.com/javascript-data-grid/column-spanning/
handsontable: Yes https://handsontable.com/docs/javascript-data-grid/merge-cells/
glide-data-grid: Partial (Column spanning but not row) https://www.github.com/quicktype/glide-data-grid/issues/487
The row version of this is currently called "row grouping": https://github.com/statico/jsgrids/blob/main/lib/features.ts#L131-L136
I'd absolutely be open to tracking column grouping, too, but I don't have the time to go through all the libraries right now. PRs welcome!
Spanning and grouping are slightly different concepts. How many grids would need to be updated for you to consider merging this feature?
I've added a mergeCells feature which I think does the job! I haven't reviewed all libraries, though, just the ones you mentioned. glide-data-grid gets a "yes, but...." warning for the feature.