Just gave the latest Qemu a try to boot an emulated SparcStation 20 from the original Solaris install CD:

/opt/qemu-3.1.0/sparc-softmmu/qemu-system-sparc \
-M SS-20 -m 256 \
-nographic \
-monitor telnet::3001,server,nowait \
-cdrom /mnt/iso/sol-8-hw4-install-sparc32.iso \
-boot d

The resulting output:

Configuration device id QEMU version 1 machine id 65
Probing SBus slot 0 offset 0
Probing SBus slot 1 offset 0
Probing SBus slot 2 offset 0
Probing SBus slot 3 offset 0
Probing SBus slot 15 offset 0
Invalid FCode start byte
CPUs: 1 x TI,TMS390Z55
UUID: 00000000-0000-0000-0000-000000000000
Welcome to OpenBIOS v1.1 built on Oct 5 2018 08:20
Type 'help' for detailed information
Trying cdrom:d...
Not a bootable ELF image
Loading a.out image...
Loaded 7680 bytes
entry point is 0x4000
bootpath: /iommu@f,e0000000/sbus@f,e0001000/espdma@f,400000/esp@f,800000/sd@2,0:d
switching to new context:
SunOS Release 5.8 Version Generic_108528-29 32-bit
Copyright 1983-2003 Sun Microsystems, Inc. All rights reserved.
\
panic[cpu0]/thread=f0244020: BAD TRAP: type=9 (Data fault) rp=f0243624 addr=0 mmu_fsr=126 rw=1 occurred in module "unix" due to a NULL pointer dereference

qemu: fatal: Trap 0x29 while interrupts disabled, Error state
pc: f0163298 npc: f016329c
%g0-7: 00000000 f00c220c 3024a52c f024fe14 4000020d f0243b88 00000000 f0244020
%o0-7: 00001fff 00001000 00001000 f5c89000 00000000 f5918240 f02500a0 f0074d94
%l0-7: fbcf4834 f013cc18 00000001 f026eb30 f5917c60 f02727e0 f5917c40 00001010
%i0-7: f5c89000 00001000 00000001 00000000 00000000 f5917c68 f0250100 f013d4a4
%f00: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
%f08: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
%f16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
%f24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
psr: 40000fc5 (icc: ---- SPE: SP-) wim: 00000020fsr: 00000000 y: 00000000

Everything works fine though, if configured for the default SparcStation 5 (SS-5). I also tried other machine types: The only working configuration is the SS-5.

I guess there is some issue with the device tree.