You are passing the value in the wrong format. You need to replace all the special codes like %3A and %2B with corresponding characters.
%3A is :
%2B is -
The below request is working without any issues.
http://localhost/ACU20193/entity/Default/20.200.001/StockItem?$top=5&$filter=LastModified gt datetimeoffset'2021-01-02T00:58:57.837-08:00'
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