We've got nothing similar to Jira in place and are dealing with things without any dedicated tools
Are you not tracking issues at all? I very frequently look up years old tickets to know why something is the way it is, and I could not imagine working without it.
That's what comments in code and project/product and technical documentation are for. Work tracking tickets should contain as little text as possible. Ideally the contents would be a brief 1-3 sentence description of the work, links to the relevant parts of the product/project and technical documentation, and whatever the definition of done is (e.g. "success measures", "acceptance criteria", and so on).
For normal work I simply don't allow my team to create tickets in JIRA unless there is at least a technical design document which contains at minimum a bullet-list describing the complete scope of work and some documentation or links to documentation regarding the specific requirements. For bugs I require the ticket created and put in our queue to be updated with the same material once the cause of error and the fix are identified.
