Look at Options tab for an image, formula, or chart -- notice that there is a "Text alternative" field. This field is not available in the Options tab for "Frame", but should be.
Confirmed. Strange that it doesn't. Adding the .uno:ObjectTitleDescription to a Frame object's context menu gives an active 'Text alternative' field on the dialog that the object accepts as input. Version: 7.5.2.2 (X86_64) / LibreOffice Community Build ID: 53bb9681a964705cf672590721dbc85eb4d0c3a2 CPU threads: 8; OS: Windows 10.0 Build 19045; UI render: default; VCL: win Locale: en-US (en_US); UI: en-US Calc: threaded