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A fix has been made to the Interrupts/Exceptions Quick Reference to clarify that mstatus.mpp is set to the the least-privileged supported mode on mret. The local version of Machine-Level ISA has been updated to the Priv-v1.12 source as this is the current version on References to the depreciated user mode interrupts and the n extension were also removed.

From mstatus:

An MRET or SRET instruction is used to return from a trap in M-mode or S-mode respectively. When executing an xRET instruction, supposing xPP holds the value y, xIE is set to xPIE; the privilege mode is changed to y; xPIE is set to 1; and xPP is set to the least-privileged supported mode (U if U-mode is implemented, else M). If xPP≠M, xRET also sets MPRV=0.

From sstatus:

When an SRET instruction (see Section [otherpriv]) is executed to return from the trap handler, the privilege level is set to user mode if the SPP bit is 0, or supervisor mode if the SPP bit is 1; SPP is then set to 0. 11