Is DD4T v2.2.1 compatible with DXA 1.8 (.NET), for a Web 8.5 environment? I would think yes as the DD4T component is on the CM side and DXA is CD, but haven't tested.
DD4T.TridionTemplates v2.2.1 was released March 2017, and v2.2.2 was released Feb 2018. DXA 1.8 was released Nov 2017.
Updating to DD4T 2.2.2 would be preferable so as to be on the latest version, but as this version was released after DXA 1.8 not sure if there are any compatibility issues there.
== Edit: Added question context ==
The prod 8.5 environment is intermittently seeing the issue described in forum link, on a running instance. This issue in turn was linked to a DD4T issue
For the DD4T issue, it looks like the suggested action is to:
- Update the Newtonsoft.Json package to the latest version, which is 11.0.
- Update to the latest version of DD4T templates.
- Republish pages.