The side connector lasted until Socket 370 in the Pentium III product cycle. The new socket took Pentium III chips that had integrated L2 cache (versus earlier PIIIs which had cache on an external chip).
Pentium 4 never had a side connector model, but did have a “socket shrink” in the transition from Willamette (0.18 u) to Northwood (0.13 u) cores. Socket 478 was kept until the Prescott cores (0.09 u). The Prescott cores were also offered with an LGA 775 socket later.
Yup, that's why I said "until Pentium4". P6.0/P6S actually introduced multi-die packages at Intel, not the SECC. Well, actually that's not true either, because I worked on Sidewinder: a 486 that fit in a 386 socket with an ASIC on the top of the package that matched the busses. So it was technically "multi-die".
Pentium 4 never had a side connector model, but did have a “socket shrink” in the transition from Willamette (0.18 u) to Northwood (0.13 u) cores. Socket 478 was kept until the Prescott cores (0.09 u). The Prescott cores were also offered with an LGA 775 socket later.