It was found that internal linkage suppresses the effect of the IRAM_ATTR attribute.

dok-net · dok-net · commit 369a0bb0436c · 2020-10-22T19:13:13.000+02:00
This affects the unnamed namespace or static non-member functions.
diff --git a/cores/esp8266/core_esp8266_wiring_digital.cpp b/cores/esp8266/core_esp8266_wiring_digital.cpp
@@ -117,7 +117,7 @@ extern "C" {
     typedef void (*voidFuncPtr)(void);
     typedef void (*voidFuncPtrArg)(void*);
 
-    namespace
+    namespace detail
     {
         struct interrupt_handler_t {
             interrupt_handler_t()
@@ -241,7 +241,7 @@ extern "C" {
 
 };
 
-namespace
+namespace detail
 {
     void ICACHE_RAM_ATTR set_interrupt_handlers(uint8_t pin, Delegate<void(), void*>&& userFunc, uint8_t mode)
     {

Original file line number	Diff line number	Diff line change
`@@ -117,7 +117,7 @@ extern "C" {`
`117`	`117`	`typedef void (*voidFuncPtr)(void);`
`118`	`118`	`typedef void (voidFuncPtrArg)(void);`
`119`	`119`
`120`		`- namespace`
	`120`	`+ namespace detail`
`121`	`121`	`{`
`122`	`122`	`struct interrupt_handler_t {`
`123`	`123`	`interrupt_handler_t()`
`@@ -241,7 +241,7 @@ extern "C" {`
`241`	`241`
`242`	`242`	`};`
`243`	`243`
`244`		`-namespace`
	`244`	`+namespace detail`
`245`	`245`	`{`
`246`	`246`	`void ICACHE_RAM_ATTR set_interrupt_handlers(uint8_t pin, Delegate<void(), void*>&& userFunc, uint8_t mode)`
`247`	`247`	`{`