**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

airflow/serialization/serialized_objects.py
if op_extra_links_from_plugin and "_operator_extra_links" not in encoded_op:
                setattr(op, "operator_extra_links", list(op_extra_links_from_plugin.values()))
        for k, v in encoded_op.items():
            if k == "_downstream_task_ids":
                v = set(v)
            elif k == "subdag":
                v = SerializedDAG.deserialize_dag(v)
            elif k in {"retry_delay", "execution_timeout"}:
                v = cls._deserialize_timedelta(v)
            elif k in encoded_op["template_fields"]:
                pass
            elif k.endswith("_date"):
                v = cls._deserialize_datetime(v)
            elif k == "_operator_extra_links":
                if cls._load_operator_extra_links:
                    op_predefined_extra_links = cls._deserialize_operator_extra_links(v)
                    
                    op_predefined_extra_links.update(op_extra_links_from_plugin)
                else:
                    op_predefined_extra_links = {}
                v = list(op_predefined_extra_links.values())
                k = "operator_extra_links"
            elif k == "deps":
                v = cls._deserialize_deps(v)
            elif k in cls._decorated_fields or k not in op.get_serialized_fields():
                v = cls._deserialize(v)
            
            setattr(op, k, v)
        for k in op.get_serialized_fields() - encoded_op.keys() - cls._CONSTRUCTOR_PARAMS.keys():
            setattr(op, k, None)