Sorry I'm coming so late to the party, so to speak, on this one. I agree with pretty much all of the comments above.
Individuals who are no longer practicing with a firm or authorized to do so may, generally, receive two types of payments, return of capital, which are generally not taxable, and amounts they are owed as their share in receivables when they leave, if the partnership has a provision for the latter. The latter can be formulaic based on aggregate receivables and/or represent an interest in specific matters, e.g. contingent fees. Amounts paid to the departing partner for their share of the receivables on the books as of the date they ceased to work for the firm , which may occur over a multi-year period, are generally fully taxable as self-employment income. Furthermore, they are, generally, payment for services performed.
Before the 2007 changes to the 415 regs, I thought that it was probably permissible, if the plan did not have a last day of the year rule for receiving an allocation, to make an allocation for the departing partner based on all of the self-employment income they received during the year of their departure. And even if the plan did have a last day of the plan year condition for receiving allocations, it's usually not completely clear that a departed partner is not "employed" on the last day of the year, when you look at the plan's wording of the self-employment provisions, which usually just say that partners are treated as employees and their self-employment income is treated as W-2 compensation. They are, after all, still receiving self-employment income all the way through the end of the year, and probably in future years as well. I even concluded that, as odd as this sounds, you arguably could make a contribution for a departed partner in the plan year after they had left, as long as they still had self-employment income from the partnership and the allocation was not inconsistent with the plan terms.
With the 2007 changes to the 415 regs, the partner's ceasing to be actively involved in the partnership's provision of services should probably be treated as a "severance from employment" under Treas. Reg. 1.415(a)-1(f)(5) for purposes of the Treas. Reg. 1.415(c)-2(e) rules regarding post-employment compensation. However, the regs are not completely clear on this point, since they do not specifically address the special issues of self-employed individuals and merely use the term "severance from employment" and then say that when that occurs is based on "facts and circumstances." To the best of my knowledge, the issue has not been specifically addressed in any guidance interpreting the regs.
Probably the best course of action is to specifically treat a departing partner as having had a "severance from employment" on the date they stopped working for the partnership, and apply the plan's rules for post-employment compensation. But the specifics of how you should address this should probably be written into the plan document, or if that is not practical, then adopted by the firm as the plan administrator as its interpretation of the plan's provisions on this point.