If doing image or video export, enabling Multisample AA
(multisample anti-aliasing, MSAA) will at least give you anti-aliasing on the clipping attachment edge:
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Eg, here is spineboy coming out of the portal without MSAA (with the sparkles removed):
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And with 8x MSAA:
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If using a runtime, you may be able to enable MSAA for your runtime. Anti-aliased clipping edges will look nicer, but if you want actual feathering then the only option is what Erikari mentioned.