What is it the strange CPU instruction generated by SDCC?

Just causes it to EAT the next byte/instruction. JRF (Jump if FALSE, ie NEVER) will take the next byte as the intended target, which it never acts upon. Think of it as the first byte of a two byte NOP, equivalent in this situation to a JRA 00106$ without the pipeline hit, and uses less code

478 ;	-----------------------------------------
 479 ;	 function USART1_RX_TIM5_CC_IRQHandler
 480 ;	-----------------------------------------
 00011F 481 _USART1_RX_TIM5_CC_IRQHandler:
 00011F 4F [ 1] 482 	clr	a
 000120 62 [ 2] 483 	div	x, a
 484 ;	/home/master/workspace/_myrtos-newview-0.0.0/SPL/stm8l15x_tim5.h: 1996: tim5_flag_l = (uint8_t)(TIM5->SR1 & (uint8_t)(TIM5_FLAG));
 000121 C6 53 06 [ 1] 485 	ld	a, 0x5306
 000124 A4 04 [ 1] 486 	and	a, #0x04
 000126 97 [ 1] 487 	ld	xl, a
 488 ;	/home/master/workspace/_myrtos-newview-0.0.0/SPL/stm8l15x_tim5.h: 1997: tim5_flag_h = (uint8_t)(TIM5->SR2 & (uint8_t)((uint16_t)TIM5_FLAG >> 8));
 000127 C6 53 07 [ 1] 489 	ld	a, 0x5307
 490 ;	/home/master/workspace/_myrtos-newview-0.0.0/SPL/stm8l15x_tim5.h: 1999: if ((uint8_t)(tim5_flag_l | tim5_flag_h) != 0)
 00012A 9F [ 1] 491 	ld	a, xl
 00012B 4D [ 1] 492 	tnz	a
 00012C 27 03 [ 1] 493 	jreq	00105$
 494 ;	/home/master/workspace/_myrtos-newview-0.0.0/SPL/stm8l15x_tim5.h: 2001: bitstatus = SET;
 00012E A6 01 [ 1] 495 	ld	a, #0x01
 496 ;	/home/master/workspace/_myrtos-newview-0.0.0/SPL/stm8l15x_tim5.h: 2005: bitstatus = RESET;
 000130 21 497 	.byte 0x21
 000131 498 00105$:
 000131 4F [ 1] 499 	clr	a
 000132 500 00106$:
 501 ;	../stm8l15x_it.c: 363: if (TIM5_GetFlagStatus(TIM5_FLAG_CC2) == SET) os_yield();
 000132 4A [ 1] 502 	dec	a
 000133 26 05 [ 1] 503 	jrne	00102$
 000135 CDr00r00 [ 4] 504 	call	_os_yield
 000138 20 06 [ 2] 505 	jra	00108$
 00013A 506 00102$:
 507 ;	../stm8l15x_it.c: 365: os_handle_interrupt_error(28);
 00013A 4B 1C [ 1] 508 	push	#0x1c
 00013C CDr00r00 [ 4] 509 	call	_os_handle_interrupt_error
 00013F 84 [ 1] 510 	pop	a
 000140 511 00108$:
 512 ;	../stm8l15x_it.c: 366: }
 000140 80 [11] 513 	iret