You could manually install the latest package, according to 
https://forum.proxmox.com/threads/meltdown-and-spectre-for-newbie.39183/#post-194316
Depending on cpu, it could work or not. I have tried on both Jessie and Stretch, but not production servers and without performance benchmarks.
WORKING
	
	
	
		Code:
	
	
		# dpkg -l | grep intel-microcode
ii  intel-microcode                        3.20180108.1                       amd64        Processor microcode firmware for Intel CPUs
#dmesg | grep microcode
[    0.000000] microcode: CPU0 microcode updated early to revision 0xc2, date = 2017-11-16
# ./spectre-meltdown-checker.sh
Spectre and Meltdown mitigation detection tool v0.29
Checking for vulnerabilities against running kernel Linux 4.4.98-4-pve #1 SMP PVE 4.4.98-104 (Mon, 15 Jan 2018 09:34:49 +0100) x86_64
CPU is Intel(R) Xeon(R) CPU E3-1275 v5 @ 3.60GHz
....
CVE-2017-5715 [branch target injection] aka 'Spectre Variant 2'
* Mitigation 1
*   Hardware (CPU microcode) support for mitigation:  YES
*   Kernel support for IBRS:  YES
*   IBRS enabled for Kernel space:  YES
*   IBRS enabled for User space:  NO
* Mitigation 2
*   Kernel compiled with retpoline option:  NO
*   Kernel compiled with a retpoline-aware compiler:  NO
> STATUS:  NOT VULNERABLE  (IBRS mitigates the vulnerability)
....
	 
 
NOT WORKING
	
	
	
		Code:
	
	
		# dmesg | grep microcode
[    0.000000] microcode: microcode updated early to revision 0x1c, date = 2015-02-26
# ./spectre-meltdown-checker.sh
Spectre and Meltdown mitigation detection tool v0.31
Checking for vulnerabilities against running kernel Linux 4.13.13-5-pve #1 SMP PVE 4.13.13-36 (Mon, 15 Jan 2018 12:36:49 +0100) x86_64
CPU is Intel(R) Xeon(R) CPU E3-1220 V2 @ 3.10GHz
VE-2017-5715 [branch target injection] aka 'Spectre Variant 2'
* Mitigation 1
*   Hardware (CPU microcode) support for mitigation
*     The SPEC_CTRL MSR is available:  NO
*     The SPEC_CTRL CPUID feature bit is set:  NO
*   Kernel support for IBRS:  YES
*   IBRS enabled for Kernel space:  NO
*   IBRS enabled for User space:  NO
* Mitigation 2
*   Kernel compiled with retpoline option:  NO
*   Kernel compiled with a retpoline-aware compiler:  NO
> STATUS:  VULNERABLE  (IBRS hardware + kernel support OR kernel with retpoline are needed to mitigate the vulnerability)
	 
 
NOT WORKING
	
	
	
		Code:
	
	
		# dpkg -l | grep microcode
ii  intel-microcode                    3.20180108.1                         amd64        Processor microcode firmware for Intel CPUs
# dmesg | grep microcode
[    0.000000] microcode: CPU0 microcode updated early to revision 0x19, date = 2013-06-21
# ./spectre-meltdown-checker.sh
Spectre and Meltdown mitigation detection tool v0.29
Checking for vulnerabilities against running kernel Linux 4.4.98-4-pve #1 SMP PVE 4.4.98-104 (Mon, 15 Jan 2018 09:34:49 +0100) x86_64
CPU is Intel(R) Core(TM) i7 CPU 950 @ 3.07GHz
...
CVE-2017-5715 [branch target injection] aka 'Spectre Variant 2'
* Mitigation 1
*   Hardware (CPU microcode) support for mitigation:  NO
*   Kernel support for IBRS:  YES
*   IBRS enabled for Kernel space:  NO
*   IBRS enabled for User space:  NO
* Mitigation 2
*   Kernel compiled with retpoline option:  NO
*   Kernel compiled with a retpoline-aware compiler:  NO
> STATUS:  VULNERABLE  (IBRS hardware + kernel support OR kernel with retpoline are needed to mitigate the vulnerability)
	 
 
But, I repeat, those are not production servers, they have booted all right, I didn't notice obvious issues. But, everybody says they are unstable, and are waiting for retpoline mitigation.