42 years hands in the Automotive work place including 6 years with a full time engine shop. Did my apprenticeship in the 1970's on Mercedes Benz (all alloy headed) later 14 years in a Ford dealer (all alloy headed inline 6 and alloy headed OHC 4 cyl) drifted around then had 6 years at Salisbury Engines here in South Australia. Yep Aluminum heads (can) go soft when they've been hot - seen so called reconditioned heads that have had weld repairs to corroded water jackets that you can't torque down because the bolts sink through the head material.
The last car maker to use iron heads here in Australia would have been GM until 6 or 7 years ago other wise I can't think of any iron headed factory engines in our market since the early 1980's maybe even the later 1970's. Can't believe you haven't seen soft heads regularly.
Cheers